This invention relates to electron multipliers of the kind in which a charge current is amplified by passage to and by secondary emission of electrons from surfaces of a dynode array, and to dynode arrays for such electron multipliers. In one aspect, the invention is particularly concerned with electron multipliers in which there are two generally parallel rows of said dynodes in said array, the dynodes in each row being in side-by-side position, and said successive dynodes being dynodes in alternating ones of said rows, each said dynode being shaped such that electric potentials which are applied to the dynodes of the array generate an electric field between the two rows such as to effect substantial direction of secondary electrons produced at each said surface to the surface of the next successive dynode; in which said dynodes in one said row have the said surface thereof facing the other said row of dynodes and those of the other said row have the said surface thereof facing said one row; each said dynode having first and second flanges bounding respective opposed margins thereof, said first and second flanges extending transverse to the directions of extent of said rows, the flanges of dynodes in said one row extending from the surfaces of those dynodes towards the other said row and the flanges of dynodes of said other row extending from the surfaces of those dynodes towards said one row, and each adjacent pair of said dynodes in said one row and in said other row having the first flange of one dynode of the pair adjacent and spaced from the second flange of the other dynode of the pair, and the first flange of each said dynode being closer to an input end of the array than the second flange of that dynode; the surfaces of dynodes in said one row and of dynodes in said other row being linear and parallel to the lengthwise direction of extent of the respective row, when the array is viewed in lengthwise cross-section normal to tangents to said surfaces at the location of the cross-section.
Our Australian Patent Specifications AU 39194/78 and AU 87312/75 describe electron multipliers of the above kind. In these constructions, the aforementioned first flange is of generally L-shaped configuration having a first portion which extends normally to the respective dynode surface and an outwardly extending second portion which extends away from the edge of the first portion remote from the dynode surface in a plane parallel to the dynode surface. The second flange extends normally from the associated dynode surface.
While the arrangements of patent specifications AU 39194/78 and AU 87312/75 have proven to be highly satisfactory in use, the two portion form of the first flanges renders the manufacture of the dynodes less simple than would be desirable. In accordance with a first aspect of the invention, this difficulty is avoided in that the first flange of each said dynode extends at an obtuse angle to the surface of that dynode, to extend away from that surface towards said input end, and the second flange of each said dynode extends at an acute angle to the surface of that dynode, from the junction of the flange with that surface, towards said input end.
In order to secure effective operation of electron multipliers, the surfaces of the dynodes to which the charge current is directed must be selected so as to have good emissive characteristics in the sense that a high secondary electron yield is obtained on incidence of an electron.
The performance of emissive surfaces deteriorates during long term use of dynodes, so that the lifetime of an electron multiplier has been, hitherto, usually limited to the lifetime of the emissive surfaces. In Australian Patent Specification AU 39194/78, a construction of cylindrical multiplier is disclosed in which the emissive surfaces are defined on strip metal which is removable from the dynodes so as to provide a construction where the life of the multiplier can be rendered indefinite by replacement of the strips as necessary. However, it is necessary in that construction to pre-form the strips to a specific configuration, such as by some suitable hand or machine process involving cutting as well as bending. Also the method of retention of the strips has rendered their replacement less simple than would be desirable. In another aspect, then, the invention seeks to provide a construction which, whilst providing the advantages of removable dynode surfaces, permits of a simplified removal and replacement procedure. In this aspect, then, the invention provides a dynode array for an electron multiplier in which a charge current is amplified by passage to and secondary emission of electrons from surfaces of successive dynodes of the array, characterised in that the emission surfaces of the dynodes are defined by foils removably positioned on supporting portions of the respective dynode. In a preferred form of the invention the foils are aluminum foils which have, unexpectedly, been discovered to have good secondary emission characteristics.